1. Field of the Invention
This invention relates to a device for reducing the noise in picture signals and, more particularly, to a noise reduction device for reducing the noise in picture signals by taking advantage of field correlation or frame correlation.
2. Description of the Related Art
In video signal transmission or recording/reproducing apparatus, such as a TV receiver or a VTR, a noise reducing device for processing video signals with a noise reducing operation by taking advantage of the field correlation or frame correlation is utilized for obtaining high-quality video signals.
Such noise reducing device is provided with a memory having a storage capacity corresponding to a one-field or one-frame period. The difference between the input video signal and the video signal delayed by one field or by one frame from the input video signal is detected as noise. The detected output is multiplied by a noise-reducing coefficient and the resulting product is subtracted from the input video signal to produce a noise-reduced video signal.
With such noise reducing device, since the step of the noise-reducing effect is compulsorily changed over from outside, only a fixed noise reducing effect may be achieved for changes in the image or picture, so that a noise-reducing effect cannot be adapted to the actual state of the picture or the residual image.
On the other hand, if the noise reducing effect is enhanced, residual image tends to be left in the picture portions where changes in the motion are acute, so that a picture which is only hard to view can be produced. Conversely, if the residual image is inhibited, it becomes impossible to achieve the noise-reducing effect. Consequently, it is extremely difficult to set the characteristics of the noise reducing device so as to achieve the noise reducing effect in such a manner as to eliminate the residual image.